Wavelength-division multiplexing has recently become popular, and for this, an optical transmitter has been employed that outputs optical signals having different wavelengths. This optical transmitter employs photodiodes (PDs) to individually monitor the optical output from each laser diode (LD) which is light source, and thereby provides feedback control for maintaining, at a predetermined level, the intensity of the light output from each LD. Specifically, the current provided for driving the LDs is controlled according to a current (hereinafter referred to as a photocurrent) output from the PDs corresponding to the optical output from the LDs.
A photocurrent monitoring circuit, illustrated in FIG. 1, is conventionally employed as an optical transmitter that includes a plurality of LDs for outputting optical signals having a plurality of different wavelengths (see, for example, non-patent document 1). Logarithmic amplifiers LA1 to LAn, which convert photocurrents into voltages, are respectively connected to photodiodes PD1 to PDn, which monitor the optical output from LDs. The output of the logarithmic amplifiers LA1 to LAn is transmitted to a processor CPU via analog-digital converters ADC1 to ADCn. The processor CPU then sequentially reads, each 250 μs, the output of the analog-digital converters ADC1 to ADCn, and calculates the drive current values for the individual LDs. The processor CPU thereafter employs the obtained drive current values to control the drive circuit for the LDs.
A problem that has been encountered, however, is that although the logarithmic amplifiers LA1 to LAn are expensive, and one such amplifier must be provided for each photodiode, as a result, the costs involved in producing a photocurrent monitoring circuit becomes exceedingly high.
The objective of the present invention is to provide a low cost photocurrent monitoring circuit.    Non-patent Document 1: E. J. Newman, “Logarithmic Processing Applied to Network Power Monitoring”, National Fiber Optic Conference, 2002 Tech. Proc. p. 1292-1298